The control of pregnancies in heifers that are surplus to breeding requirements is an important component of cattle management in the extensive beef production systems of northern Australia. Particularly significant is the prevention of pregnancies in tum-off heifers and cast-for-age cows. Traditionally, fertility control has been achieved by surgical procedures that include paralumbar (flank) and vaginal (passage) spaying.
More recently, the Willis Drop-Ovary Technique has been introduced as an alternate surgical procedure. The latter would appear to 'be the preferred surgical approach, but requires a relatively high level of technical proficiency. A challenge for the beef industry is to identify a contraceptive technology for heifers and cows that is non-invasive, practical, can be readily applied, and satisfies animal production imperatives whilst recognising the importance of consumer and community awareness of animal welfare.
A potential non-surgical technology for contraception in female cattle is the use of a gonadotrophin releasing hormone (GnRH) agonist bioimplant. The reproductive hormone cascade in cattle is initiated by the release of endogenous GnRH from the base of the brain. GnRH acts at the anterior pituitary gland to stimulate a pulsatile pattern of release ofluteinising hormone (LH) and follicle stimulating (FSH), which is necessary for normal ovarian function. Agonists of GnRH prevent the pulsatile release of LH and FSH and this leads to suppressed ovarian follicular growth and failure to ovulate.
Recently, a GnRH agonist bioimplant was developed which releases microgram quantities of GnRH agonist (deslorelin) in a controlled, sustained manner (Peptech Animal Health Pty Limited). During preliminary testing in Project NAP3.105, prototype GnRH agonist bioimplants induced contraceptive responses in heifers and cows for periods ranging from 200 to 350 days. In the latter studies, a return to normal ovarian function and pregnancies in treated animals coincided with increased pasture availability and accelerated rate of live weight gain. This observation raised the question of whether a GnRH agonist bioimplant can maintain suppressed ovarian function in heifers and cows that are undergoing a relatively fast rate of live weight gain. The present project was therefore specifically designed to address this question.
A second objective was to identify a dose of GnRH agonist (deslorelin) that would induce a contraceptive effect in heifers for 12 months, or longer. Sexually mature heifers (26-month-old) were randomly divided into 4 groups and, in January 1998, received one of the following treatments: Group I (n = 50), control, no treatment; Group 2 (n = 50), GnRH agonist Low Dose (3 mg bioimplant); Group 3 (n = 50), GnRH agonist Medium Dose (6 mg bioimplant); Group 4 (n = 48), GnRH agonist High Dose (12 mg bioimplant). Heifers were maintained on pasture together with 4 herd bulls, and were monitored for appropriate rates of live weight gain. Ovarian activity and pregnancy status were monitored at monthly intervals using ultrasonography .